The problem to which the invention is directed will be explained based on signal transmission between a transmitter-side (in general, a superordinated unit) and a sensor-side (in general, a consumer). The invention can, however, be applied in other systems, where data and energy need to be transmitted.
Usually attached to a transmitter is a cable for connection to a sensor. The cable connection to a sensor occurs frequently via a plugged connection, for example, via a galvanically decoupled, especially inductive, interface. In this way, electrical signals can be transmitted contactlessly. The galvanic isolation leads to advantages as regards corrosion protection, potential isolation, preventing mechanical wear of the plug, etc. Such systems are sold by Endress+Hauser under the mark “Memosens”.
The signal to be transferred, i.e. the data (the “information”), must be converted into a suitable format for transmission. In this regard, a so-called carrier signal is changed by the data. This procedure is called “modulation”. The opposite procedure, thus the filtering out of data from a carrier signal, is called “demodulation”.
Binary transmission of digital signals occurs in the simplest case by using a two state, rectangular signal. In this case, there is a switching between two amplitudes, frequencies or phases. The present invention relates to the use of an amplitude modulation. In the transmission of digital signals, one speaks of keying instead of modulation, in which sense the invention relates to ASK, or amplitude shift keying.
The mentioned inductive interface is usually embodied as a system with two coils, which are, for example, plugged into one another. Typically, both data (in two directions) as well as also energy (from transmitter-side to sensor-side) are transmitted. The energy must, in such case, be large enough that a connected sensor is supplied sufficiently with energy, so that a durable measurement operation is assured.
The challenge of such contactless energy- and data transmission lies, first of all, in the rough operating- and environmental conditions in industrial environments. Because of the environmental conditions (temperature, humidity, etc.), tolerances for the components (tolerances of inductances of the coils, etc.) are especially large. If, for example, assemblies are designed for typical temperatures, with which medical equipment is sterilized (typically above 120° C.), then this means e.g. that the coils experience significantly modified inductance values at high temperatures.
Of concern relative to the tolerances is especially the coupling transformer, which inductively couples the coil on the transmitter-side with the coil on the sensor-side, respectively forms with these two coupled coils a transformer. In the case of this coupling system, the mechanical pairing of the two partner coils is less exactly toleranced than in the case of usual transformers. In other words, this contactless energy- and data transmission system involves very large deviations of the inductive coupling and especially large deviations of the inductance values.
Another challenge in the industrial environment involves power limiting, which enters especially in the case of explosion-protection requirements. Explosion protection requirements mean, for instance, limiting power to below 10 mW.
Moreover, it must be assured that data are transmitted uncorrupted. Two effects must be taken into consideration as error sources for the corruption of data signals.
On the one hand, non-uniformity of the components can lead to corrupted transmissions. If, for example, the inductance value of the coil on the transmitter side is detuned relative to the nominal frequency, significantly higher or significantly lower signal levels, i.e. especially voltage level, result for the data signal, since, among other things, the resonance circuit formed by the coil is detuned. In the one case, there is the danger that the transmitter-side attempts to retrieve on the supply lines more power than is available. In the case of too low signal levels, the transferred energy is insufficient for reliably operating the sensor-side.
On the other hand, it is possible that the amplitude of the alternating signals applied on the transmitter- and sensor sides can be influenced by load jumps of the sensor. Thus, there results for the case, in which the sensor has for short-times an increased power requirement, the danger that the resulting jumps in signal level are incorrectly interpreted as a communication signal.